The problem of condensation on mirrors is well known. In warm and humid environments such as bathrooms, ventilation fans are used to exhaust humidity from such environments, but their effect is slow, and is wasteful of heat energy in cold climates.
Mirror condensation can be eliminated by heating the mirror directly. A common approach is to use built in resistive electrical heating elements to provide such heat such as is shown in U.S. Pat. Nos. 3,160,736 and 4,857,711.
The Spencer patent, U.S. Pat. No. 4,665,304, discloses a sheet-like heating element that can be mounted behind a sheet of conventional mirror glass. While the Spencer heating element may be attached to unmounted conventional mirrors, it is not easily retrofitable to an installed mirror. As shown in FIG. 8 of the Spencer patent, a transformer 20 in a junction box 19 and a cable enclosure 15 protrude from the rear of the mirror heater, necessitating cutouts in the wall on which the mirror is mounted. Installation, therefore, is difficult and expensive. The Spencer apparatus is not usable where holes may not be cut in the wall (i.e. in rental units and sites using masonry construction).